U.S. Pat. Nos. 3,935,099, 3,981,100, 3,985,616, and 3,997,484, all issued in 1976, describe the formation of a class of materials referred to as “super absorbent polymers” (SAPs). Since 1976, many patents relating to the use and formation of SAPs have issued. While most of these patents disclose compositions made by copolymerizing acrylic acid and acrylamide in the presence of a coupling agent, a few patents describe the use of a natural polymer, such as a starch (see, e.g., U.S. Pat. Nos. 3,935,099, 3,981,100, 3,985,616, and 3,997,484). SAPs made without starch are referred to as “totally synthetic copolymers.” Almost all such SAPs are used in baby diapers, adult diapers, catamenials, hospital bed pads, cable coating, and the like. Today the market for totally synthetic copolymer SAPs is estimated to be about 2 billion pounds per year worldwide.
Starch graft copolymer compositions with the ability to absorb up to 1,000 times their weight in aqueous fluids are known in the prior art. The prior art disclosed cross-linked starch graft copolymers that absorb large quantities of aqueous fluids for use in absorbent soft goods, that increase the water-holding capacity of soils, and that act as coatings on seeds, fibers, clay, paper, and the like. The prior art also disclosed methods for creating SAP films by drying the composition in trays or by heating it on drum dryers. These films can then be ground or milled into flakes or powders. In an alternative prior art drying method, a viscous mixture of alkali starch graft copolymer was diluted with a water-miscible organic solvent such as alcohol or acetone. The precipitated alkali starch graft copolymer was then isolated in a fine powder form by filtration and drying.
Agricultural companies that market seed, fertilizer, herbicides, insecticides, and other agricultural materials have found little use for the totally synthetic copolymer SAPs in agriculture because evaluations of these SAPs show poor performance because they are composed of small, fine particles having an 80 mesh size. One inherent limitation of finer-mesh particles is that they cannot be used in typical granule applicators which require particle sizes of at least 25 mesh. Further, the SAP films and powders cannot be applied with granular fertilizers, granular pesticides, or other granular agricultural additives.
The present inventors have recognized a need for a method of producing a granular SAP product for use in large-scale agricultural applications.